Making paper or textile products soft without impairing performance characteristics such as strength or absorbency has long been the goal of various workers. Softness is the tactile sensation perceived by a person who holds a particular paper or textile product and rubs it across the skin. Such tactilely-perceivable softness can be characterized by, but is not limited to, friction, flexibility, and smoothness, as well as subjective descriptors, such as a feelings of lubriciousness, or softness textures reminiscent of velvet, silk, or flannel. However, improvement of softness in almost all cases comes at the expense of strength or absorbancy of the fibrous material.
One method for improving softeness in paper products is to select or modify cellulose fiber morphologies to those which provide advantageous microstructures. However, while incorporation of upgraded cellulose fiber sources into paper products can improve softness, it is often the case that upgraded fiber sources offer limited ability to confer the properties of durability and absorbency to paper products produced therefrom, and the resulting paper products are typically possessed of the best achieveable balance between softness and strength for the treatment method or system utilised.
Another area that has received a considerable amount of attention in improving paper softness is the addition of chemical softening agents to the fiber furnish during the papermaking process. For example, chemical softening agents can be applied to the paper web during its formation either by adding the softening agent to the vats of pulp which will ultimately be formed into a paper web, to the pulp slurry as it approaches a paper making machine, or to the wet paper web as it resides on a Fourdrinier cloth or dryer cloth on a papermaking machine. In addition, the chemical softening agent can be applied to a finished paper web after it has dried.
To ensure an optimum level of softening efficiency in general, a high degree of attraction of the chemical softening composition to the fibers used in the manufacture of papers is necessary. It has been known that, because of their charge, cationic softeners have a strong affinity for the papermaking fibers and are a good softener. In comparison, anionic debonders, because they have the same charge as the fiber, are not sufficiently retained on the fiber furnish to function effectively as softeners. In addition, anionic debonders contribute to wet-end deposition and significant foaming that is in general overall detrimental to the papermaking process. Nonionic surfactants have no ionic attraction for the fibers whatsoever, and as a result, when nonionics are employed it is necessary for them to be applied to the wet paper web.
During the papermaking process, cationic debonders, when employed, are typically added to water to make an emulsion, and then added to the fiber furnish. Unfortunately, addition of cationic debonders to the fiber furnish often results in a significant reduction of strength in the paper web (strength being the ability of the paper product, and its constituent paper webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions). This reduction in strength is believed to result from a disruption of hydrogen bonds between the papermaking fibers that are formed as a result of the papermaking process. In order to offset the effects of the strength reduction that occurs because of the cationic debonder addition, dry strength additives must be added; however, these additives often negate the softness benefits imparted by the cationic debonder addition.
Various compositions are known in the art as being useful for conferring softness to paper products For example, published U.S. Patent Application No. 20020112831 discloses a paper softening composition containing a quaternary ammonium compound, water, and a nonionic surfactant. Other compositions and methods for paper softening are disclosed in U.S. Pat. Nos. 6,458,343; 6,369,007; 6,315,866; 6,245,197; 6,200,938; 6,179,961; 6,004,914; 5,753,079; 5,538,595; 5,385,642; 5,322,630; 5,240,562; 4,959,125; 4,940,513; 4,720,383; 4,441,962; 4,351,699; and 3,554,862, the entire contents of which aforesaid patent documents are herein incorporated by reference thereto in their entirety.
One of the most important physical properties related to softness is generally considered by those skilled in the art to be the strength of the paper web. Accordingly, there is a continuing need for soft paper and textile products having good strength properties. There is also a need for improved softening compositions that can be applied to such paper and textile products to provide the requisite softness without unacceptably degrading the strength of the product.